To assure reliability of operation, stored program controlled telephone switching systems are conventionally designed to employ some degree of on-line maintenance testing as part of the switching control function as well as off-line diagnostic testing to locate the source of trouble once a unit has been taken out of service. An early system of this type is the No. 1 ESS described in the September 1964 issue of the Bell System Technical Journal. That system's maintenance plan, described at pp. 1961-2019, included duplicated main processors, each of which performed a number of classes of base level work. Maintenance testing of the duplicated processors was periodically performed in class E of the base level wherein the states of corresponding internal processor nodes were compared either on a routine basis or on a directed or sampled mode basis under program control. Maintenance of the network, however, was limited to checking that one and only one path selection relay operated in the on-line one of the duplicated network controllers, i.e., no direct checking of a network path itself could be accomplished.
Another example of the use of on-line maintenance followed by off-line diagnostic testing is shown in W. Schurter U.S. Pat. No. 3,609,704 issued Sept. 28, 1971. In that patent a diagnostic routine is described for isolating a "babbling"memory store from among a plurality of memory stores accessible over a common address bus.
Recently, a useful technique for off-line diagnosis has been developed employing an iterative encoding device known as a signature accumulating chip (SAC) device. See, for example, E. White, "Signature Analysis--Enhancing the Serviceability of Microprocessor-Based Industrial Products," IECI '78 Proceedings, pp. 68-76; R. A. Frohwerk, "Signature Analysis: A New Digital Field Service Method," Hewlett-Packard Journal, May 1977,pp. 2-8; and H. J. Nadig, "Signature Analysis-Concepts, Examples and Guidelines", ibid, pp. 15-21. The operation of the SAC device is premised upon the provision of a predetermined stimulus to a circuit or series of circuits which terminate at the SAC device. When the SAC device has had an opportunity to respond to the stimulus, the SAC device is connected to a register and the contents of the register compared with an anticipated response word. If all of the intervening circuitry operates correctly, an equal comparison should result. The nature of any disagreement between the register and the anticipated response word can be indicative of the nature of the fault giving rise to the disagreement. Because the SAC device requires a predetermined stimulus to elicit an anticipated response whereas the stimuli (voice or data messages) traversing communications switching network paths are generally not predictable, it has not heretofore been possible to employ SAC chip devices to perform on-line testing of communications switching paths.